Yersinia pestis, the causative agent of plague, is a highly pathogenic organism that spreads rapidly and causes extremely high human mortality. Although it is sensitive to a restricted panel of antibiotics, its extraordinary pathogenicity, the potential weaponization of aerosolized bacteria with bio-engineered antibiotic resistance, and the lack of an effective vaccine or therapy are major concerns, contributing to its classification as a Tier 1 Biothreat Agent. The outer membrane protein Ail is a Y. pestis virulence factor and a prime candidate for therapeutic development due to its two principal activities: (i) mediating the adhesion of Y. pestis bacteria to host cells, and (ii) providing resistance to human complement. This project focuses on understanding its adhesion activity as a step towards its development as a novel therapeutic target. The working hypothesis is that Ail binds one or more host proteins, triggering host intracellular signaling cascades and the translocation of bacterial factors into the host cell that block phagocytosis and enable bacteria to survive extracellularly. Adhesion is the first key step in this series of events; however, how does Ail mediate bacterial adhesion to human cells? Molecular information is lacking. The specific aims of this project are designed to fill this glaring gap by obtaining functional and structural information about the adhesion activity of Ail and the interactions with its human host partners.